The present invention relates generally to projection optics used in semiconductor manufacturing equipment, and particularly to a catadioptric projection optic system having a high numerical aperture used with short wavelengths.
In semiconductor manufacturing, photolithography techniques are often used. These photolithography techniques require the image of a reticle to be projected onto a wafer or photosensitive substrate. Relatively complicated projection optics are often used to project the image of the reticle onto the wafer or photosensitive substrate. The projection optics are required to provide a very high quality image of the reticle so that very small feature sizes on the reticle can be imaged accurately with very little aberrations. The projection optics often provide a magnification less than one resulting in a reduced image. Often, only a small portion of the image field is utilized that has the best imaging qualities. However, it is desirable to provide as large an image field as possible to enhance throughput and increase production of semiconductor devices. With the tremendous demand for decreased feature sizes in combination with higher throughput, new and improved projection optical systems are continually needed. Because of the ever decreasing feature sizes demanded by the semiconductor manufacturing industry, projection optics are needed that have higher numerical apertures and that are designed to operate at shorter wavelengths. Current optical designs cannot meet the demands of the manufacturers of semiconductors. For example, a prior optical system is disclosed in U.S. Pat. No. 4,953,960 entitled xe2x80x9cOptical Reduction Systemxe2x80x9d issuing Sep. 4, 1990 to Williamson. Therein disclosed is an optical reduction system operating in the wavelength range of 248 nanometers and having a numerical aperture of 0.45. Another projection optical system is disclosed in U.S. Pat. No. 5,089,913 entitled xe2x80x9cHigh Resolution Reduction Catadioptric Relay Lensxe2x80x9d issuing Feb. 18, 1992 to Singh et al, which is herein incorporated by reference. Therein disclosed is an optical system having a restricted spectral wavelength at 248 nanometers and having a numerical aperture of 0.6. Another projection optics system is disclosed in U.S. Pat. No. 5,537,260 entitled xe2x80x9cCatadioptric Optical Reduction System With High Numerical Aperturexe2x80x9d issuing Jul. 16, 1996 to Williamson, which is herein incorporated by reference. Therein disclosed is a projection optics system having a numerical aperture of 0.7 with different embodiments operating in wavelengths ranging from 360 to 193 nanometers. While these optical systems have operated adequately, there is a need for a projection optics used in semiconductor manufacturing to reproduce feature sizes substantially smaller than those of current systems.
The present invention comprises a catadioptric optical system using multiple aspheric surfaces improving performance and reducing the number of lens elements. Calcium fluoride lens elements are used in a lens group closest to the wafer or photosensitive substrate. A zero-order quarter waveplate is positioned after the reticle and before a lens group having at least one aspheric surface prior to a beamsplitter. An aspheric concave mirror is placed adjacent the beamsplitter and adjacent a surface perpendicular to the lens group. Another lens group is positioned adjacent the beamsplitter opposing the aspheric concave mirror having a majority of the lens elements made of calcium fluoride and imaging the reticle at the wafer or photosensitive substrate. A relatively high numerical aperture of 0.75 is obtained, and in one embodiment a wavelength of 157 nanometers is utilized.
Accordingly, it is an object of the present invention to provide a projection optic system with a higher numerical aperture than current projection optic systems.
It is a further object of the present invention to decrease the lens elements of the projection optic system.
It is yet a further object of the present invention to prevent asymmetry in reticle diffraction caused by polarized illumination.
It is an advantage of the present invention that it results in reduced aberrations.
It is a further advantage of the present invention that reduced feature sizes can be imaged.
It is yet a further advantage of the present invention that it uses circular polarized electromagnetic radiation through the reticle.
It is a feature of the present invention that it uses calcium fluoride as a lens material in a lens group near the wafer.
It is another feature of the present invention that multiple aspheric lens elements are used.
It is yet another feature of the present invention that a zero-order quarter waveplate is positioned after the reticle.
These and other objects, advantages, and features will be readily apparent in view of the following description.